The present invention pertains to the field of processing acoustic signals, and more particularly to the field of measurements of the speed of sound in a medium of unknown constituents when the direction of propagation of the sound is known, such as when sound propagates in a fluid within a conduit.
In extracting oil and gas from a formation, it is advantageous to monitor the flow rates of the different components of the production fluid, usually gas, oil and water. It has been established that measuring the sound speed of a mixture can be used to determine the volumetric phase fractions of the components since the speed of sound in a mixture can be directly related to the speed of sound in the components of the mixture.
Techniques for determining the speed at which a pressure disturbance travels along an array of sensors have been developed for use in many fields, such as the fields of sonar processing, radar, and seismic imaging. For example, in the field of underwater sonar signal processing, a technique called beam forming is used to determine the direction of approach (DOA) of an acoustic signal based on determining the speed at which the acoustic wave travels along the array. Knowing the speed of sound in the water and the speed at which the acoustic wave travels along the array enables the determination of the direction of approach of the acoustic signal. Many different processing techniques have been developed for use in such applications, techniques aimed at extracting from an array of sound detectors the speed at which a wave travels across an array of sensors. (See, e.g. xe2x80x9cTwo Decades of Array Signal Processing Researchxe2x80x94the Parametric Approach,xe2x80x9d by H. Krim and M. Viberg, IEEE Signal Processing Magazine, pp. 67-94.)
In contrast to underwater sonar applications, in a production fluid flowing through a conduit, sound-producing disturbances occur continuously, as a natural consequence of the flow of the production fluid through the conduit, and their locations are not of interest. Therefore, in measuring the speed of sound in such a conduit in order for example to use the value of the speed of sound for some monitoring function, it is not necessary to provide a source of sound. Moreover, again in contrast to underwater sonar applications, the direction of travel of the essentially one-dimensional, planar sound waves within a conduit is known, i.e. the sound is either traveling upstream or downstream within a conduit. Thus, the problem of measuring the speed of sound in a fluid contained within a conduit has known values for a principal unknown of a sonar application, namely the direction of approach, but has as an unknown what is assumed in a sonar application, namely the speed of sound.
What is needed in many applications, including determining the speed of sound in a fluid within a conduit, is a way of adopting the methodologies of underwater sonar signal processing to what is essentially the inverse of the problem solved in that field, i.e. using information provided by an array of sound detectors to determine not the direction of approach to a sound source relative to the axis of the array in a 3-dimensional medium of known sound speed, but instead using the array of sensors to directly measure the speed of sound within a conduit in which the direction of approach is known to be aligned with an axis of the array.
Accordingly, the present invention provides a method and corresponding system for measuring the speed of sound in a fluid contained within an elongated body, the sound traversing the elongated body substantially along a direction aligned with the longest axis of the elongated body, the sound causing a momentary change in pressure in a portion of the fluid as the sound traverses the portion of the fluid, the method including the steps of: providing at predetermined locations an array of at least two sensors distributed along the elongated body, each sensor for discerning and signaling spatio-temporally sampled data including information indicating the pressure of the fluid at the position of the sensor; acquiring the spatio-temporally sampled data from each sensor at each of a number of instants of time; constructing a plot derivable from a plot, using a technique selected from the group consisting of spectral-based algorithms, such as the Capon method or the MUSIC method, in which a spectrum-like function of the speed of sound is formed, and parametric methods of solution, such as the deterministic maximum likelihood method; identifying in the plot a spectral ridge, and determining the slope of the spectral ridge; and determining the speed of sound assuming a relation between the speed of sound and the slope of the spectral ridge.